The present invention relates to a spindle motor to be mounted to, for instance, a hard disc drive, an optical disk drive, a magneto-optical disc drive, a magnetic disc drive and a polygon mirror. More particularly, it relates to a fluid bearing of the spindle motor and a method of assembling the bearing.
Recently, an HDD device similar devices to which a spindle motor is mounted have been downsized conveniently for hand carrying. However, a user will sometimes happen to drop the device carelessly or hit the device against another device by mistake due to the smaller size, the lighter weight, or due to hand carrying.
Various kinds of spindle motors are available in the market, and thus a major component of the spindle motor (i.e., a fluid bearing device) has a number of forms. Many of the fluid bearing devices include a cylindrical sleeve, which bears the load of a shaft in a radial direction. The sleeve surrounds the shaft via an annular space between an outer wall of the shaft and an inner wall of the sleeve. This space is called xe2x80x9ca radial bearing spacexe2x80x9d and is filled with lubricant such as oil. When a rotor-hub fixedly mounted to a first end of the shaft rotates, hydrodynamic-pressure-generating grooves (e.g. herringbone grooves) generate dynamic pressure in the lubricant, so that a radial bearing is formed. The grooves are provided on the outer wall of the shaft and the inner wall of the sleeve.
A disc-shaped thrust plate is fixed to a second end of the shaft, and herringbone grooves, for instance, are provided on at least one of an upper face or a lower face of the thrust plate. Lubricant is filled into the grooves, so that a thrust bearing is formed in order to bear a load in an axial direction.
A conventional brush-less motor having a fluid bearing is disclosed in Japanese Patent Application Non-Examined Publication No. 2000-50567 (hereinafter referred to as xe2x80x9cprior art 1 xe2x80x9d), and this motor features excellent shock resistance. Another Japanese Patent Application Non-Examined Publication No. H11-280755 (prior art 2) discloses a spindle motor featuring improved abrasion resistance as well as shock resistance. Still another Non-Examined Publication No. 2000-274428 (prior art 3) discloses a spindle motor which includes a fluid bearing having a shaft member with a flange. This flange is formed by press-fitting a ring member to be used in dynamic pressure into a cylindrical member. This prior art 3 publication also discloses a method of assembling the spindle motor. Another Non-Examined Publication No. H05-161382 (prior art 4) discloses a disc driving device which prevents the accuracy of whirling of a pulley or a rotor yoke from lowering. The pulley or the rotor yoke positions and retains a disc. To achieve this advantage, a projection is formed on a rear side of the rotor yoke at a vicinity of an inner rim. The projection closely faces an end face of a bearing for a rotary shaft. The rotor yoke is spot-welded to the rotary shaft or welded by laser at a vicinity of the inner rim of the front surface.
Prior art 1 teaches that a flange is arranged on a sleeve on its rotor hub side, and a pull-stopper fixed to the rotor hub is brought into contact with the flange when the rotor hub moves along the shaft. These mechanics thus require a complicated structure. Prior art 2 teaches that a thrust plate is fixed to an end of a sleeve by caulking, thereby forming a sleeve having a bottom made from the thrust plate in order to increase shock resistance. The caulked section is sealed with adhesive. Therefore, prior art 2 does not teach anything about a flange to be fixed to a shaft. Prior art 3 teaches that the cylindrical member (i.e., a shaft of the spindle motor) and the ring member are machined independently, then the cylindrical member is press-fitted into the ring member to form an integrated shaft structure. However, the press-fitting invites scratches on the ring member and the shaft, and it must be admitted that this structure has a limited mechanical strength. Instead of machining the shaft and flange independently, a structure unitarily formed with a shaft and a flange and having a cross-shaped sectional view would overcome the problem discussed above. However, this shaft would boost the production cost, and it would be difficult to obtain satisfactory accuracy of a shaft diameter. It is extremely difficult to provide grooves for producing dynamic pressure on the shaft section and the flange section of the structure having a cross-shaped sectional view. This will lower the productivity and boost the production cost. Prior art 4 teaches that the surface of a pulley is spot-welded or laser-welded to the shaft at the vicinity of the inner rim. However, prior art 4 does not disclose anything about the structure of the spindle motor or how to improve the mechanical strength between the shaft and the disc-shaped flange to be fixed to the shaft. Thus, improvement of the shaft""s service life and shock resistance of the spindle motor cannot be expected.
The object of the present invention is to provide a spindle motor including the following elements: a shaft; a disc-shaped flange fixedly mounted to a first end of the shaft and having a diameter greater than that of the shaft; a cylindrical sleeve to be fit to the shaft; a radial bearing space formed between an outer wall of the shaft and an inner wall of the sleeve closely facing the outer wall of the shaft; a rotor hub fixed to a second end of the shaft and facing an upper end of the sleeve; a thrust plate closely facing the flange; a thrust bearing space formed between the faces, closely facing each other, of the flange and the thrust plate; and lubricant retained in the thrust bearing space. The disc-shaped flange has a hollow section around its center in which the shaft is fit, and the second end of the shaft is inserted into the hollow section such that the shaft is stopped while being inserted into the hollow section. As a result, a recess is formed in the hollow section. This insertion stopping of the shaft in the hollow section does not stuff up the hollow section, but leaves the recess therein. This recess is useful for recognizing a welding section between the shaft and the flange.
The recess left in the hollow section can be used as a place to be welded (i.e., a margin for applying the welding). Laser can be irradiated to the inner wall of the hollow section using this recess, so that a welding section of the flange can be within a thickness of the flange. A bending moment caused by shrinkage stress due to melting/hardening becomes so small that the flange is not deformed into a dish-shape, and the flange thus keeps its flatness and vertical status excellent. As a result, the motor features a long-life shaft. The recess in the hollow section can also work as a reservoir of lubricant. This reservoir has a sufficient room for the lubricant and is located near the thrust bearing space, so that the shaft can enjoy a longer service life. A wall of the reservoir can be made seamless by laser welding, so that the reservoir is kept highly airtight. Therefore, if a spiral groove, where a center reservoir receives the highest pressure, is formed and the thrust bearing space having a uniform width is prepared, a high load bearing capability can be expected. As a result, the shaft can enjoy a longer service life.
A spindle motor of the present invention comprises the following elements: a shaft, a disc-shaped flange fixedly mounted to a first end of the shaft and having a diameter greater than that of the shaft; a cylindrical sleeve to be fit to the shaft; a radial bearing space formed between an outer wall of the shaft and an inner wall of the sleeve closely facing the outer wall of the shaft; a rotor hub fixed to a second end of the shaft and facing an upper end of the sleeve; a thrust plate closely facing the flange a thrust bearing space formed between the faces, closely facing each other, of the flange and the thrust plate; and lubricant retained in the thrust bearing space. The shaft includes a larger diameter section and a smaller diameter section at a first axial end which forms a projection, and the flange includes a hollow section at a vicinity of a center thereof. The smaller diameter section of the projection is fit to the hollow section. The hollow section has a smaller diameter instead of a greater diameter which would be needed to receive the greater diameter section of the shaft. This smaller diameter restrains the flange from reducing its active area, and the capability of the thrust bearing is not lowered. As a result, the thrust bearing can bear a large load, and the shaft has a longer life. The radius of the flange is greater than the thickness of the flange. Thus, when an inner wall of the hollow section is welded by laser, the flange exerts a great resistance against deformation due to shrinkage stress, thereby minimizing the deformation. As a result, the motor enjoys a longer shaft life.
A method of assembling the spindle motor of the present invention comprises the following steps: preparing a shaft; bringing a first disc-shaped unit into contact with the shaft, the first disc-shaped unit being a part of a thrust bearing for bearing an axial load of the shaft, and fixing the disc-shaped unit to the shaft; welding the contacting face between the disc-shaped unit and the shaft by laser; fitting the shaft to a cylindrical sleeve; and facing a second disc-shaped unit, which forms the thrust bearing, closely to the first disc-shaped unit. According to this method, almost all of the contacting face between the shaft and the first disc-shaped unit is welded by laser, so that the fixing strength between the shaft and the first disc-shaped unit, e.g., a flange, can be enhanced. As a result, the spindle motor features stronger shock resistance.
Another method of assembling the spindle motor includes: fixing a flange to a first axial end of a shaft, the flange having a greater diameter than the shaft; inserting the second axial end of the shaft into a cylindrical and open-ended sleeve, thereby extending the second axial end from the second end of the sleeve; facing a first face of the flange closely to a first face of a thrust plate; and fixing the thrust plate to a part of the first end of the sleeve, thereby sealing the first end of the open-ended sleeve. According to this method, a unitized component of the spindle motor can be assembled, so that the shaft, flange, thrust plate and sleeve are integrated into one unit. As a result, a complicated assembly process is simplified using this unitized component. When the flange is fixed to the end of the shaft, the thrust plate, which jointly forms the thrust bearing with the flange, can be fixed with ease. Since the open-ended sleeve is used, the shaft with the flange can be fit to the sleeve with ease.